Our troops are daily crossing and advancing, but all is otherwise quiet. We are now receiving an issue of clothing, which we greatly need. Our ranks are putting on a new-revived appearance. The first sergeants of the Harris Light have received orders to finish their pay-rolls. General Lee is reported to be falling back to the Rappahannock. _Sunday, July 19._--Our cavalry left Harper's Ferry at two o'clock P. M., crossed the river on pontoons at Sandy Hook, and advanced into Virginia. Monthly returns for June were made before our march commenced. The weather is very warm and sultry. On the twentieth we resumed our march at ten A. M., and advanced to Leesburg, where we fed our horses and rested. In the decline of the day we marched to Goose Creek, on whose grassy banks we bivouacked for the night. The whole cavalry force is moving towards the Rappahannock. On the twenty-first we advanced _via_ Gum Spring and Centreville to Manassas Junction. The boys have had some gay times to-day after blackberries, which we found in great abundance all along our line of march. General Gregg was compelled to dismount several men in the forenoon, and ordered them to march on foot, for the offence of leaving the ranks for berries, without permission. A command would soon be totally demoralized, if such tendencies to unsoldierly conduct were not checked. And though at times discipline seems severe, yet, especially with us, it is absolutely necessary. _July 22._--To-day we marched to the vicinity of Gainesville. We fell in with Scott's Nine Hundred as we were marching across the old field of Bull Run, among whom we found several old acquaintances. We spent a few very interesting moments together. _July 23._--Our command was cheered to-day by the arrival of a large mail, which brought a message to nearly every man. During active campaigning, as in the invasion of Pennsylvania and Maryland, it is difficult to keep up postal connections with the civil world, and, with the very best efforts which can be made, our mails are greatly delayed, sometimes even for weeks together. But when they do come, they are hailed with a delight which is almost frantic. The post-boys are cheered as far as they can be seen, as they wend their way from camp to camp, with their horses loaded down with the enormously swollen mail-bags. Several bushels of letters are sometimes brought by one carrier, as was the case to-day. FIGHTING AT MANASSAS GAP.